Introduction
A footing is only as good as what goes into the trench: the concrete mix, the steel, the chairs holding that steel off the dirt, and the formwork shaping it all. Each material has a job — concrete carries compression, rebar carries tension, chairs guarantee the two actually work together — and each has a wrong choice that quietly undermines the footing. This guide covers what to order, in what sizes, and in what quantities.
The Concrete Footing Calculator converts your footing’s dimensions into concrete volume — cubic meters, cubic yards, and bag count. This guide covers everything else on the materials list: mix strengths, rebar sizes and laps, chairs and cover, formwork, and a worked quantity takeoff with metric and imperial cross-checks.
Concrete Mixes for Footings
Footing concrete lives underground in constant contact with soil moisture, so it needs adequate strength and durability — but it doesn’t need the high strengths specified for suspended slabs or columns. The standard residential footing mix:
| Application | Typical strength (metric) | Typical strength (imperial) |
|---|---|---|
| Garden walls, light non-structural footings | 20 MPa | ~3,000 psi |
| House strip footings (reinforced) | 20–25 MPa | ~3,000–3,600 psi |
| Retaining walls, poor drainage, freeze-thaw exposure | 25–32 MPa | ~3,600–4,600 psi |
Check the conversions: 20 MPa × 145.04 = 2,901 psi ≈ 3,000 psi; 25 MPa = 3,626 psi; 32 MPa = 4,641 psi. When ordering ready-mix, tell the plant it’s a footing pour and confirm the mix — they batch footing concrete daily and will flag anything unusual about your specification. For bagged mixes on small runs, the bag states its 28-day strength; match it to the table above.
Two mix cautions specific to footings. First, don’t over-water at the trench — every extra litre weakens the concrete permanently, and footing concrete is hard to inspect after burial. Second, in sulfate-rich soils or high water tables, ask the supplier about sulfate-resistant cement; standard mixes degrade faster in aggressive ground. Pricing the concrete once you’ve chosen the mix is covered in the cost guide.
Rebar: Sizes, Placement, and Laps
Residential strip footings typically carry two continuous longitudinal bars, one near each face of the strip. The bars run the full length of the footing, lapped at joints and bent continuously around corners.
| Bar designation | Diameter (metric) | Diameter (imperial) | Typical footing use |
|---|---|---|---|
| #3 | 10 mm | 3/8 in | Light garden-wall footings |
| #4 | 12–13 mm | 1/2 in | Standard house strip footings (2 bars) |
| #5 | 16 mm | 5/8 in | Heavier loads, wide footings (2–3 bars) |
| #6 | 19 mm | 3/4 in | Engineered footings, retaining walls |
Bars are sold in 6 m (20 ft) lengths, so a 40 m run needs joints — and every joint needs a lap splice of about 40 bar diameters: 0.48 m (19 in) for 12 mm bars (0.48 × 39.37 = 18.9 in). Stagger the laps on the two bars so they don’t align at one cross-section, and use L-shaped corner bars to carry steel continuously around turns. The lap math is derived in the formula guide.
Chairs, Spacers, and Concrete Cover
Rebar only works when it’s embedded in concrete — steel touching soil corrodes, and corroded steel loses both section and bond. The requirement is 75 mm (3 in) minimum concrete cover between steel and ground on all sides: below, at the sides, and at the ends.
- Chairs (plastic or concrete) hold the longitudinal bars up off the trench bottom. Space them every 1.0–1.5 m (3–5 ft) along each bar line — a 40 m run needs roughly 30–40 chairs per line, 60–80 total for two bars.
- Side cover comes from keeping the bars off the trench walls: in a 0.6 m (24 in) trench with two bars, position them roughly 75 mm (3 in) clear of each face.
- Never use bricks, stones, or wood blocks as chairs — they absorb water, degrade, or create weak inclusions. Purpose-made chairs cost little and don’t compromise the pour.
- Keep the steel clean. Light surface rust is harmless, but mud, oil, or heavy scale weakens the steel-to-concrete bond. Hose down bars that got dirty during trench work.
Formwork: When the Trench Is the Form
The cheapest formwork is firm ground itself. In stable, cohesive soil, trenches are dug to the footing’s width and concrete is poured directly against the earth — no form boards needed. Formwork becomes necessary when:
- The soil is loose or sandy and trench walls won’t stand vertical.
- The footing must be wider than the trench (poured above grade or in a stepped profile).
- Step faces on sloped sites need shaping — each step’s riser gets a form board.
- Ends, corners, or construction joints need a clean stop face.
Form boards are typically 25 mm (1 in) lumber or plywood, braced against the trench walls. Oil or release agent on the boards keeps them from bonding to the concrete. Whatever the setup, measure the concrete’s dimensions for the calculator — formwork defines the pour, and the pour is what you pay for.
Worked Example: Full Materials Takeoff
The standard example — 40 m run, 0.6 m wide, 0.3 m deep, straight trench in firm soil, 10% concrete waste:
Concrete: 40 × 0.6 × 0.3 = 7.2 m³; × 1.10 = 7.92 m³ (10.36 yd³). Cross-check: 131.23 × 1.97 × 0.98 = 254.3 ft³ = 9.417 yd³ × 1.10 = 10.36 yd³. Ready-mix at 20–25 MPa (3,000–3,600 psi) — or 466 eighty-pound bags if bag-mixing, which at this volume is impractical.
Rebar: 2 bars × 40 m × 1.05 (lap allowance) = 84 m (276 ft) of 12 mm (#4) bar — cross-check: 84 × 3.281 = 275.6 ft. In 6 m lengths: 84 ÷ 6 = 14 bars. Lap length 0.48 m (19 in) at each joint.
Chairs: every 1.2 m along two bar lines: (40 ÷ 1.2) × 2 = ~67 chairs. Tie wire: one small coil is plenty for a run this size.
Formwork: none for the straight trench walls (poured against firm earth); allow a few boards for the two end stops and any step faces. The estimating guide turns this takeoff into an order list with waste factors for each line.
Related Guides
- Concrete Footing Complete Guide — strip footing sizing, trench dimensions, and the full worked example.
- Concrete Footing Estimating Guide — quantity takeoff: concrete, steel, excavation, and waste.
- Concrete Footing Cost Guide — turning the materials list into a budget.
- Concrete Footing Pro Tips — placing and curing these materials correctly on pour day.
- Concrete Footing FAQ — quick answers to the most asked footing questions.
- Concrete Footing Checklist — verify materials on site before the truck arrives.
Frequently Asked Questions
What strength concrete should I use for footings?
Typically 20–25 MPa (3,000–3,600 psi) for residential strip footings; 25–32 MPa (3,600–4,600 psi) for retaining walls or freeze-thaw exposure. Confirm the mix with the ready-mix plant when ordering.
What size rebar goes in a strip footing?
Two continuous 12 mm (#4, 1/2 in) bars is the residential norm, one near each face. Heavier loads or wider footings step up to 16 mm (#5) bars or three bars — an engineer’s call.
How far apart should rebar chairs be in a footing?
Every 1.0–1.5 m (3–5 ft) along each bar line. A 40 m run with two bars needs roughly 60–80 chairs to maintain the 75 mm (3 in) minimum cover.
Can I pour footing concrete directly against the soil?
Yes, in firm, stable soil with vertical trench walls — the ground is the formwork. Loose or sandy soil needs form boards to hold the trench shape.
How long should rebar lap splices be in footings?
About 40 bar diameters: 0.48 m (19 in) for 12 mm (#4) bars. Stagger laps between the two bars and carry steel continuously around corners.
Do I need special concrete for wet or sulfate soils?
In sulfate-rich soils or high water tables, ask the supplier about sulfate-resistant cement — standard mixes degrade faster in aggressive ground. It’s a cheap upgrade against a buried, uninspectable element.
Order It Right
The materials list for a strip footing is short — concrete at the right strength, two runs of bar with laps, enough chairs for 75 mm cover, and form boards only where the soil demands them. The Concrete Footing Calculator nails down the biggest line item, the concrete volume, from your run length, width, depth, and waste allowance. Take its number plus the steel and chair counts above to your supplier, and the order writes itself.
Disclaimer: Calculator results are planning estimates based on the dimensions you enter. Actual material requirements vary with soil conditions, local frost depth, trench accuracy, and supplier practices. Verify quantities with your supplier and consult a licensed professional or your local building authority for structural or code-related decisions.
